So last week I cross threaded a park plug. Not too deep, but probably two full revolutions. As any sane person would, I freaked out. I looked at thread chases but they seemed like you could mess it up even more, and push metal shavings into the head.

So after some searching I found the

Powerbuilt 640811 14mm Back-Tap Thread Repair Tool

Basically you insert the tool into the head, expand the threads and back it out.

Even with the threads retracted you can lightly thread it in. Once reasonable deep I expanded it and then backed out. I probably did over a dozen passed. I was being extremely careful. I also kept the tool covered in grease to catch metal filings

The results were, I was able to easily thread my plug all the way down by hand before torquing it down

These don't appear to available in stores so I had to have Rover down all weekend while I waited for the tool.

I've used this before, bought mine from Amazon. Cyl #8 spark plug threads on a D2 were bunged up, and the spark plug was only about 2 threads in (forced/cross threaded) when I got it. Trying to chase the threads from the outside was hopeless, as they were destroyed. This thing was slick as hell, just want to reiterate that you need to grease it up to catch all the filings that come off. 3-4 graduated passes did the trick for me.

Ditto. Saved money, time and trouble. i also bought the $25 Amazon one. However, there are better ones which might be worth it now that i know the experimental effort worked so well instead of Helicoiling or more intensive options.